Britain cannot afford to see the creation of an "extreme Islamist regime" in the middle of Iraq, David Cameron has said.
The Prime Minister said that the militants of the Islamic State of Iraq and the Levant (Isis) threatening the government in Baghdad were also plotting terror attacks on the UK.
Mr Cameron, who is due to chair a meeting of the National Security Council to discuss the crisis in Iraq, said returning militants from the fighting in the Iraq and neighbouring Syria were now assessed to represent a greater threat than those from Afghanistan.
While Britain supported efforts of Iraqi prime minister Nouri al-Maliki to tackle extremism in his country, he said that it was essential that the Iraqi government represented the interests of all the people and not just the Shia majority.
"I disagree with those people who those people who think this is nothing to do with us and if they want to have have some sort of extreme Islamist regime in the middle of Iraq, that won't affect us. It will," Mr Cameron told MPs at Prime Minister's questions.
"The people in that regime - as well as trying to take territory - are also planning to attack us here at home in the United Kingdom.
"So the right answer is to be long-term, hard-headed, patient and intelligent with the interventions that we make.
"The most important intervention of all is to make sure that these governments are fully representative of the people who live in their countries, they close down the ungoverned space, and that they remove the support for the extremists."
Mr Cameron said Britain was increasing the humanitarian aid it was sending to people displaced by the fighting in Iraq from £3 million to £5 million.
He said it was "vital" that Isis was pushed back by the forces of the Iraqi government after the sweeping gains it made last week.
In the UK, he said, security, intelligence and policing resources were focused on the Middle East region and the dangers of British people travelling there, becoming radicalised and returning home.
"The estimates are now that this is a greater threat to the UK than the return of foreign jihadis or fighters from the Afghanistan or Pakistan region," he said.
Mr Cameron stressed that British assistance to the rebels in Syria - where Isis is also engaged in fighting the forces of president Bashar Assad - had been directed to the official opposition.
He said that other countries in the region, which have been accused of supplying more extremist elements, should do the same.
"Our engagement with the Saudi Arabians, with Qataris, with Emiratis and others is all on the basis that none of us should be supporting those violent terrorists or extremists," he said.
Downing Street later revealed that up to 14 people have had their passports seized to stop them going to Syria to fight over the past year.
The total of 14 seizures between April 2013 and March 2014 related to people suspected of links with terrorism and extremism or serious and organised crime, but the Prime Minister's official spokesman said "a significant number of them" were Syria-related.
Meanwhile, some 65 Syria-related arrests have been made by police in the UK over the 18 months since January 2013 - including 40 in the first quarter of this year.
The spokesman was unable to say how many of these arrests were linked to people planning to travel to Syria or to offer support to rebels in Syria, and how many related to fighters who had returned from taking part in the civil war.
Mr Cameron's spokesman said: "The use of the passport confiscation powers and the fact that arrests are being made which are directly linked to the security of UK citizens underlines the action that has been taken so far and will continue to be taken.
"What we have seen over a period of time is a shift towards a situation where the situation in Syria is the number one source of the jihadist threat, which has been a shift away from the Afghanistan and Pakistan tribal area threat. We have seen British security activity shift and become refocused in response to that."
He added: "It is absolutely right that we take preventative action to stop individuals in this country participating in extremist and terror-related activities anywhere in the globe."
According to Downing Street, Mr Cameron's conclusion at the NSC meeting this afternoon was: "The Government should continue to do all it can to address the threat posed to the UK by the return of foreign fighters, including by interviewing individuals at the UK border suspected of being involved in acts of terrorism, cancelling or withdrawing passports of suspected foreign fighters seeking to travel to Syria or Iraq, and introducing new measures to prosecute those who plan and train for terrorism abroad.
"We should press the Iraqi government to pursue an inclusive political response and that we should work with moderate Shias, Sunnis and Kurds in Iraq who support a democratic, pluralistic state with respect for the rule of law.
"UK military intervention is not on the table but we should continue to work with the United States and countries in the Gulf region to prevent the spread of violent extremism in the region.
"And we should continue to provide humanitarian support to Iraq and neighbouring countries bearing the burden of refugees fleeing both Iraq and Syria."